CSET Practice Test Subtest II Science

Vision
by Dr. Carl Bianco, M.D.
How Stuff Works.com

It's no accident that the main function of the sun at the 
center of our solar system is to provide light. Light is what 
drives life. It's hard to imagine our world and life without it. 

The sensing of light by living things is almost universal. 
Plants use light through photosynthesis to grow. Animals 
use light to hunt their prey or to sense and escape from 
predators. Some say that it is the development of 
stereoscopic vision, along with the development of the 
large human brain and the freeing of hands from locomotion, 
that have allowed humans to evolve to such a high level. 

Perceiving Light
When light enters the eye, it first passes through the 
cornea, then the aqueous humor, lens and vitreous humor. 
Ultimately it reaches the retina, which is the light-sensing 
structure of the eye. The retina contains two types of 
cells, called rods and cones. Rods handle vision in low light, 
and cones handle color vision and detail. When light 
contacts these two types of cells, a series of complex 
chemical reactions occurs. The chemical that is formed 
(activated rhodopsin) creates electrical impulses in the 
optic nerve. Generally, the outer segment of rods are long 
and thin, whereas the outer segment of cones are more, 
well, cone shaped.

The outer segment of a rod or a cone contains the 
photosensitive chemicals. In rods, this chemical is called 
rhodopsin; in cones, these chemicals are called color 
pigments. The retina contains 100 million rods and 7 million 
cones. The retina is lined with black pigment called melanin 
-- just as the inside of a camera is black -- to lessen the 
amount of reflection. The retina has a central area, called 
the macula, that contains a high concentration of only 
cones. This area is responsible for sharp, detailed vision. 

When light enters the eye, it comes in contact with the 
photosensitive chemical rhodopsin (also called visual purple). 
Rhodopsin is a mixture of a protein called scotopsin and 
11-cis-retinal -- the latter is derived from vitamin A (which is 
why a lack of vitamin A causes vision problems). Rhodopsin 
decomposes when it is exposed to light because light causes 
a physical change in the 11-cis-retinal portion of the 
rhodopsin, changing it to all-trans retinal. This first reaction 
takes only a few trillionths of a second. The 11-cis-retinal 
is an angulated molecule, while all-trans retinal is a straight 
molecule. This makes the chemical unstable. Rhodopsin breaks 
down into several intermediate compounds, but eventually 
(in less than a second) forms metarhodopsin II (activated 
rhodopsin). This chemical causes electrical impulses that are 
transmitted to the brain and interpreted as light. 

Activated rhodopsin causes electrical impulses in the following 
way: 

1. The cell membrane (outer layer) of a rod cell has an 
electric charge. When light activates rhodopsin, it causes 
a reduction in cyclic GMP, which causes this electric charge 
to increase. This produces an electric current along the cell. 
When more light is detected, more rhodopsin is activated 
and more electric current is produced. 
2. This electric impulse eventually reaches a ganglion cell, 
and then the optic nerve. 
3. The nerves reach the optic chasm, where the nerve fibers 
from the inside half of each retina cross to the other side of 
the brain, but the nerve fibers from the outside half of the 
retina stay on the same side of the brain. 
4. These fibers eventually reach the back of the brain 
(occipital lobe). This is where vision is interpreted and is 
called the primary visual cortex. Some of the visual fibers 
go to other parts of the brain to help to control eye 
movements, response of the pupils and iris, and behavior. 

Eventually, rhodopsin needs to be re-formed so that the 
process can recur. The all-trans retinal is converted to 
11-cis-retinal, which then recombines with scotopsin to 
form rhodopsin to begin the process again when exposed 
to light. 

Source:
Partial excerpt from article entitled How Vision Works by 
Dr. Bianco. For complete article visit the fascinating web 
site HowStuffWorks.com.

The Simple Bare Necessities of Life

Would it surprise you to learn that you have the same
 basic needs as a tree, a frog, or a fly? In fact, almost
 every organism has the same basic needs: food,
 water, air and living space.

Food

All living things must have food. This probably does not
 come as a surprise. Food provides organisms with the
 energy and raw materials needed to carry on life
 processes and to build and repair cells and body parts.
 But not all organisms get food in the same way. In
 fact, organisms can be grouped into three different
 categories based on how they get their food.

Some organisms, such as plants, are called producers
 because they can produce their own food. Plants are
 examples of producers. Like most producers, they use
 energy from the sun to make food from water and
 carbon dioxide. Some producers, like the microorganisms
 in Movile Cave, obtain energy and food from the
 chemicals in the environment.

Other organisms are called consumers because they
 must eat (consume) other organisms to get the food
 they need. The lizard and eagle are examples of a
 consumer. They get the energy they need by eating
 insects (lizard), small animals (eagle), or other
 organisms.

Some consumers are decomposers. Decomposers are
 organisms that get their food by breaking down the
 nutrients in dead organisms or animal wastes. Fungi
 (mushrooms) get their food by breaking down the
 nutrients in dead organisms or animal wastes. The
 fungi is absorbing nutrients from the dead plant
 material in the soil.

Water

You may have heard that your body is made up mostly
 of water. In fact, your cells and cells of almost all
 living organisms are made up of approximately 70
 percent water-- even the cells of a cactus, a camel,
 and a dragonfly. The metabolism of organisms is
 dependent on water because most of the chemical
 reactions that take place in organisms involve water.

Organisms differ greatly in terms of how much water
 they need and how they obtain it. You could survive
 for only about 3 days without water. You obtain
 water from the fluids you drink and also from the
 food you eat. The desert-dwelling kangaroo rat never
 drinks. It gets all of its water from its food.

Air

Air is a mixture of several different gases, including
 oxygen and carbon dioxide. Animals, plants, most
 other living things need oxygen in order to stay alive.
 Oxygen is used in respiration, the chemical process
 that releases energy from food. Organisms that live
 on land get oxygen from the air. Organisms living in
 fresh water and salt water either take in dissolved
 oxygen from the water or come to the water's surface
 to get oxygen from the air. Some organisms, such as
 the European diving spider, go to great length to get
 oxygen. The spider surrounds itself with an air bubble
 so that it can obtain oxygen underwater.

Green plants, algae, and some bacteria also need
 carbon dioxide gas in addition to oxygen. The food
 these organisms produce is made from carbon dioxide
 and water by photosynthesis, the process that
 converts the energy in sunlight to energy stored in
 food.

A Place to Live

All organisms must have a space to live in that contains
 all the things they need to survive. Some organisms,
 such as elephants, require a large amount of space.
 Other organisms, such as bacteria, may live their entire
 life in a single pore on the tip of your nose.

Because the amount of space on Earth is limited,
 organisms often compete with each other for food,
 water and other necessities. Many animals, including
 the bird, will claim a particular space and try to keep
 other animals away. Plants also compete with each
 other for living space and for access to water and
 sunlight.

Source:
Thurgood Marshal Middle School